This invention relates to a scanning electron microscope, and more particularly to a scanning electron microscope suitable for observing the magnetization condition of the portion of a magnetic material which is in the vicinity of its outer surface.
There are two types of magnetization distribution observing methods using a scanning electron microscope, one of which utilizes the deflection of secondary electrons in a magnetic field in the vicinity of the outer surface of a magnetic material (D. C. Joy et al, Phil. Mag. 17 (1968) 61), and the other of which utilizes the deflection of reflected electrons in a magnetic field in a sample material (J. Philibert et al, Micron 1 (1969) 174). In both of these methods, the relative differences betwen the magnetization directions in magnetic domains can be observed but it is difficult to determine the magnetization directions and the magnitude of the magnetization, the resolution of these methods being 1 .mu.m at the highest. These inconveniences occur for the following reasons.
In the former method which utilizes a stray magnetic field generated due to the magnetization of a sample, a magnetic domain structure having no stray magnetic fields cannot be observed in an extreme case. Even if a stray magnetic field exists in a magnetic domain structure, the magnetic fields, which are away from the outer surface of the sample, generated due to different magnetic domains offset each other when the magnetization distribution is dense, so that the magnetic field decreases. Consequently, it becomes difficult to deflect secondary electrons by utilizing a stray magnetic field, and magnetization distribution cannot be observed. It is not so easy to determine magnetization direction by utilizing a stray magnetic field; it is next to impossible to determine magnetization direction by utilizing secondary electrons which are deflected by a stray magnetic field.
In the latter method, the deflection of the reflected electrons due to a magnetic field in the interior of a sample is utilized to turn the intensity of the reflected electrons detected in a specific direction into video signals and thereby observe the magnetic domains. However, in order to form a clear image of a magnetic domain structure, it is necessary to increase the energy of the primary electrons. When such energy is increased, the reflected electrons in the interior of a sample expand greatly, so that resolution does not increase; the limit of the resolution in this method is 1 .mu.m.
The resolution of a regular scanning electron microscope for observing a microscopic structure with secondary electrons is determined by the probe diameter of a primary electron beam. However, when the magnetization distribution is observed by the above-described methods, the lower limit of resolution is determined by the intensity of a stray magnetic field, the intensity of a magnetic field in the interior of a sample and the degree of expansion of the reflected electrons. Accordingly, even when the diameter of the probe is reduced, resolution cannot be increased.